“This is a very heinous act,” Thornton police spokesman Victor Avila said. “We don’t know exactly what the motive of the person was, but it was certainly a terrible act.”
Shots rang out with no warning, witnesses said.

“The employees started screaming. Customers were screaming,” said Aaron Stephens, 44, who in the self-checkout line when he heard one shot and then two bursts of gunfire.
“They were running like crazy, and I ran out too because I didn’t want to get killed.”

“The Walmart girl said, ‘The balloons are popping.’ And I turn around and I just see all the people walking, falling down because they were really scared. I just told them ‘What’s going on?’ and they were like ‘They’re shooting! They’re shooting!’ ”

This Walmart in Thornton is about 23 miles from Columbine High School in Littleton, where 12 students and a teacher were slaughtered by two other students in 1999.

It’s 18 miles from the Century 16 movie theater in Aurora, where 12 people were killed in a 2012 shooting.